Pre-amps are like politics and religion.
When you try to discuss them you will almost always offend some people. So I usually just keep quiet when these discussions arise. (However, you did ask.)
Unfortunately, it's impossible to know how well (or poorly) a pre-amp will work in any given location.
The best way to find out if a pre-amp would work for you is to borrow one before you buy. Unfortunately most of us don't have a friend with good, spare pre-amps sitting around.
Other than that, you're gambling the price of the pre-amp.
Then there's the problem of risking a little money on a poor pre-amp, or more money on a good pre-amp.
The following only applies if you don't have overload or intermod problems already.
If you do have any problems without a pre-amp, then a pre-amp is not your friend.
Here are a few things to consider.
Try one.
Not just any pre-amp but one with reasonably good specs. Unfortunately many pre-amps intended for scanner use only talk about gain and not much else. The Jim M-75 pre-amp is good one to try because the gain is variable from -10 to + 20db. It has a relatively low noise figure but most other specs aren't given.
You'll want an amplifier with gain that is reasonable for your situation.
If you're only using a short feed line, or using the pre-amp at the scanner end, you DO NOT want a huge amount of gain. That would almost surely overload your radio.
So you'll want something with a low noise figure, gain that is reasonable for your location, and one with a high IP3. (The higher the IP3 the better.)
"IP3 is an industry standard figure of merit used to rate the linearity of receivers and other signal processing systems. High IP3 equals good linearity. Good linearity equals immunity to intermodulation distortion caused by strong signals on adjacent frequencies."
High IP3 is not a guarantee that a pre-amp will work for you though.
One with a 35 dBm IP3 will be less prone to problems than one with only a 10 dBm IP3.
This isn't a recommendation but here's a link to a reasonable low noise ampifier with decent specs. for an example.
LNA-1400
• Frequency Range: 100-1400MHz
• Gain: 20dB
• P1dB: +17dBm
• IP3: +35dBm
• Noise Figure: 0.8dB
http://www.rfbayinc.com/LNA/LNA-1400.pdf